Ultrasound blood flow Doppler audio with pitch shifting

ABSTRACT

An ultrasonic diagnostic imaging system produces audio Doppler from detected Doppler signals. The Doppler signals are detected in a band of frequencies which corresponds to the velocity of blood flow signals, and Doppler information is displayed based on the detected band of frequencies. The audio Doppler system produces Doppler audio in a frequency band which is shifted in pitch from the detected band of frequencies. The operator of the ultrasound system is provided with a user control by which the degree of pitch shifting can be controlled. The ultrasound system displays Doppler blood flow velocities referenced to a transmit Doppler frequency f0, with the audio Doppler being shifted in pitch from the frequencies corresponding to the blood flow velocities.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation of application Ser. No. 13/320,233,filed Nov. 11, 2011, which is a U.S. National Phase application under 35U.S.C. § 371 of International Application No. of PCT/IB2010/051712,filed Apr. 10, 2010, which claims the benefit of U.S. Provisional PatentApplication No. 61/177,673, filed on May 13, 2009. These applicationsare hereby incorporated by reference herein

This invention relates to medical diagnostic ultrasound systems and, inparticular, to ultrasound systems which produce Doppler audio which canbe controlled in pitch.

The use of Doppler audio to diagnose blood flow extends back for manydecades. In the years before real time video presentations of Dopplerflow characteristics were possible, audio was the only means ofultrasonically diagnosing blood flow. The clinician would aim theDoppler probe toward the organ or vessel of interest, unaided by video,and listen for the characteristic pulsatile “whooshing” sound of bloodflow. Since the Doppler frequency shift is generally in the kilohertzrange or lower, the amplified Doppler frequency signal could be used todirectly drive an audio loudspeaker. This remains the way thatultrasonic Doppler signals from blood flow are produced today. Thediagnostic use of audio Doppler has declined, however, as the live videopresentations of Doppler flow such as spectral Doppler and colorflowDoppler have provided more precise and spatially specific ways ofultrasonically diagnosing blood flow characteristics. Nevertheless,Doppler audio remains a staple of diagnostic ultrasound and is stillused today to help guide and confirm proper probe and sample volumeplacement. When the visual display viewed by the sonographer and theDoppler audio heard by the sonographer are both producing correspondinginformation, the sonographer's confidence in the validity and accuracyof the diagnostic information is reinforced.

The Doppler audio signal is played through a sound system which is partof the ultrasound system. In cart-mounted systems the loudspeakers ofthe sound system can be located at a variety of positions on the cart,such as in the system mainframe, on the control panel, or on thedisplay. Since the loudspeakers are carried by the cart and can bepositioned in various places, a wide variety of commercially availableloudspeakers are available for use by the system designer. But with therecent advent of more compact, hand-carried ultrasound systems, thespace for loudspeakers is much more greatly restricted. In compactsystems, size and weight are at a premium and the system designer isoften restricted to using very small, very compact loudspeakers. In theCX50 compact ultrasound system of Philips Healthcare, for example, theloudspeakers are mounted in the carrying handle, mandating the use ofvery small, very thin profile speakers. By their nature, smallloudspeakers will generally have a higher range of frequency responsethan larger speakers, an outcome dictated by both their small size andrestricted acoustic environment. These speakers are not capable of thehigher fidelity low frequency (bass) response characteristic of largerloudspeakers in larger acoustic enclosures.

As indicated above, the loudspeakers of an ultrasound system mustreproduce audio sounds of the Doppler signal frequency range. Thefrequencies of the Doppler signal are proportional to the speed orvelocity of blood flow. For relatively higher velocity blood flow, goodfidelity reproduction is generally within the capability of the smallspeakers of a portable system, as the higher frequency sounds from thehigher velocity flow are within the reproducible frequency range of thespeakers. These higher blood flow velocities are usually encounteredwhen diagnosing arterial blood flow. But in venous flow diagnosis, theblood flow velocities are much lower. Venous blood flow in the saphenousveins of the leg may be in the range of only a few centimeters persecond, for instance, or lower. Consequently the Doppler audioreproduced from these low flow rates will be low frequencies which arepoorly reproduced by small speakers. The Doppler audio will be of lowvolume, garbled, and difficult to comprehend. Hence it would bedesirable to provide better audio reproduction of the lower frequenciesencountered especially in venous ultrasound exams.

One approach to the problem of low frequency reproduction has been triedfor the tissue Doppler mode. In tissue Doppler, the motion of tissue,such as the motion of the myocardium of the heart, is detected byDoppler processing. Unlike blood flow, the motion of tissue is themovement of a continuous section of tissue, in which the tissue cellsare moving in unison since they are physically joined together. Thus,the tissue motion will be dominated by the unitary speed of the joinedtissue cells and not the range of velocities produced by turbulent bloodflow. As a result, the Doppler signal will be characterized by apredominant single frequency at any short interval of time. The meanfrequency value is used to generate a synthetic sinusoid at the meanfrequency. To reproduce the Doppler signal at a higher frequency it isonly necessary to multiply the mean frequency by a frequency scalingterm to shift the sinusoid frequency to a higher frequency. A 60 Hzsignal can be simply modulated up to 180 Hz, for instance by multiplyingthe frequency of the synthesized sinusoid by three.

While successful for tissue Doppler, this mean frequency shiftingapproach is inadequate for Doppler audio of blood flow. As mentionedabove, the blood cells in a vein or artery are disassociated and willmove in the blood flow substantially independent of each other. The flowvelocity at the center of a blood vessels will be greater than the ratealong the vessel walls due to friction at the vessel walls which isabsent in the center of the vessel. Blood cells can also move in manydifferent directions due to turbulence caused at obstructions andvalves. Consequently, blood flow is not characterized by a singlevelocity as tissue is, but by a multitude of simultaneous velocities. Itis the spectrum of audio frequencies corresponding to these velocitieswhich is produced by Doppler audio, which to a trained ear is rich inharmonics and subtle timbre. Trained sonographers rely upon thisrichness of the Doppler audio sound to guide them in probe and samplevolume placement. Simply shifting the mean frequency of the Dopplersignal to a higher frequency cannot reproduce the full spectrum ofDoppler shift frequencies arising from blood flow and will produce amonotonic, artificial sound that is unfamiliar to the trainedsonographer. Consequently a different approach must be taken to theproblem of improving the reproduction of low frequency venous flow audioDoppler sounds.

Diagnosis and use of Doppler audio requires considerable experience, asthe nuances of the complex Doppler sounds can be very subtle and arecontextually comprehended only by trained sonographers. This challengeis made more difficult by the fact that the Doppler demodulationfrequency plays a role in the Doppler audio sound. While Dopplerharmonic imaging (used primarily for tissue PW Doppler and contrastimaging) uses a demodulation frequency twice that of the transmittedfrequency so as to detect the second harmonic of the transmit frequency,conventionally the Doppler demodulation frequency is set equal to thetransmit frequency for optimal blood-flow detection in the absence ofcontrast agents. A probe which transmits and receives at 3 MHz (a 3 MHzprobe) will produce a different range of Doppler audio than a 5 MHzprobe for instance, and the sonographer will select a differentfrequency probe for different exams depending upon factors such as thedepth of the blood vessel to be examined. It would be desirable if thisdifference in Doppler frequency bands could be eliminated so that theDoppler audio would be reproduced at the same range of audio frequenciesregardless of the frequency of the probe.

In accordance with the principles of the present invention, a diagnosticultrasound system is described which produces blood flow Doppler audioat an audio frequency which is not the Doppler shift frequency. In oneimplementation the sonographer is provided with a user control thatenables adjustment of the pitch of the Doppler audio signal. The controlpreferably enables the Doppler audio sound to be shifted in pitch byfractions of an octave or by an octave or more. This is done, not bysimply shifting the mean frequency of the Doppler audio band, but bystretching or dilating the entire range of frequencies within the audioband so that the nuances of the blood flow tones are accuratelypreserved and reproduced. While the sounds of the Doppler audio areshifted in pitch, visual display of the Doppler blood flow velocities isdone at the unshifted Doppler velocity frequencies. The user control cancause the Doppler audio signal to be produced at a higher octave duringvenous flow exams with a small speaker ultrasound system, for instance.In another implementation, pitch control is used to reproduce audioDoppler in a constant frequency spectrum, thereby eliminating thedifference in Doppler audio due to the use of different frequencyprobes.

In the drawings:

FIG. 1 illustrates in block diagram form an ultrasonic diagnosticimaging system constructed in accordance with the principles of thepresent invention.

FIG. 2 illustrates an ultrasound system display screen showing aspectral Doppler display and a 2D image of the sample volume and flowcursor locations.

FIGS. 3a-3e illustrate the shifting of the Doppler audio spectrum inaccordance with the principles of the present invention.

FIG. 4 illustrates a technique for shifting the pitch of a blood flowDoppler audio which may be implemented in a sampled data system.

Referring first to FIG. 1, an ultrasound system constructed inaccordance with the principles of the present invention is shown inblock diagram form. The array transducer 12 of an ultrasound probe 10transmits ultrasonic waves and receives ultrasonic echo signals inresponse to the transmitted signals. The elements of the arraytransducer 12 are shown transmitting ultrasound beams over asector-shaped field of view 100 and along a Doppler beam axis 22. Thetransmission of ultrasound by the array transducer is controlled by atransmit beamformer 62 which controls parameters such as the frequencyof transmission and the timing of transmission by the individualelements of the array. The elements of the transducer array convert thereceived ultrasound signals to electrical signals which are transmittedby way of a transmit/receive (T/R) switch 26 to a receive beamformer 64.The receive beamformer 64 forms coherent echo signal samples S(t) fromthe signals received from the transducer elements. The transmit andreceive beamformers are synchronized and controlled by a beamformercontroller 60.

The coherent echo signals are generally received along a sequence ofbeam directions and the echo signals undergo quadrature detection by aquadrature bandpass (QBP) filter 28. A typical QBP filter is describedin U.S. Pat. No. 6,050,942 (Rust et al.) The QBP filter 28 producesquadrature I and Q components for each echo signal. These components maybe amplitude detected to form grayscale image data by a B mode processor30 using the expression √{square root over (I²+Q²)}. The I,Q componentpairs are also stored in an ensemble memory 32 in temporally differentsamples from the same image field location for Doppler processing. Theensemble memory facilitates a transform from “fast time,” which is afunction of the r.f. sampling rate, to “slow time,” which is a functionof the pulse repetition interval (PRI) at which each sample volume inthe image field is interrogated. The data ensembles are coupled to awall filter 37 which removes undesired Doppler shift components. Forblood flow imaging the wall filter 37 removes low frequency componentsfrom slow-moving tissue, and for tissue motion imaging the higherfrequency components of flowing blood are removed. The instantaneousfrequencies at different points in the image field may be color-coded incorrespondence with the frequencies (which correspond to velocities) anddisplayed in a color Doppler display by a colorflow Doppler processor38. The color Doppler display generally overlays a grayscale B modeimage for structural orientation of the color-coded motion or flow. Thecolorflow Doppler display will give the clinician a view of theinstantaneous flow or motion over the full image field, dynamicallyshown in real time.

The ultrasound system of FIG. 1 is also capable of continuous wave (CW)Doppler measurements. In CW Doppler a Doppler signal is transmittedcontinuously from one aperture of the array transducer 12 and echoes arecontinuously received by another aperture of the transducer array. Theechoes received are those from the overlap of the transmit and receivebeams. The received signals s(t) are mixed with sine and cosinefunctions of the transmit frequency and low pass filtered to removeundesired mixed components, generally the sum frequencies. Thedifference frequencies are then image processed and displayed spectrallyin the same manner as a pulsed wave (PW) spectral Doppler display.

In accordance with the principles of the present invention the receivedecho signals S(t) are phase demodulated to the Doppler shift band by agated Doppler demodulator 34. The gating demodulates echoes returnedfrom a sample volume location which may be provided by the Doppler beamvector and sample volume gating signal discussed below. The demodulatedDoppler signals I₀,Q₀ are referenced to the transmit Doppler frequencyf₀ and are of the form I(t)+jQ(t), sometimes referred to as the“analytic signal.” The demodulated Doppler signals are filtered by awall filter 35 to remove tissue components and pass only blood flowcomponents. The wall filtered blood flow Doppler signals are of the formI₀′,Q₀′. These blood flow signals are then processed for spectraldisplay by a spectral Doppler processor 36. For spectral Doppler displaythe spectral Doppler processor will produce a spectrum of frequencyvalues which correspond to the range of frequencies of blood flow thatexist at the time of the measurement. Each time-sequential spectrum isdisplayed as a spectral line in a (generally scrolling or sweeping)spectral display by the spectral Doppler processor 36 as shown in FIGS.2 and 3. The spectral Doppler display will give the clinician a detailedquantification of the flow or motion components at a specific samplevolume in the image field. The grayscale (B mode) image data, thespectral Doppler data, the CW Doppler data, and the colorflow data areall coupled to an image processor 40 for the production of one or moreimages in the desired image format(s) on a display 24.

In accordance with the principles of the present invention theultrasound system of FIG. 1 also produces an audio Doppler signal from aloudspeaker 44. Conventionally the audio Doppler signal is atDoppler-shifted frequencies based on the transmitted Doppler frequencyf₀ and produced as an audio signal since the Doppler-shifted frequenciesare in the human audible spectrum, typically 100 Hz to 10 kHz. When theDoppler audio signal is formed digitally, it is converted to an analogsignal by a digital to analog converter (DAC) 42, amplified, and appliedto the loudspeaker 44. The audio signals may also be separated as afunction of the flow direction with respect to the transducer array by aforward/reverse separator 54, which will reproduce signals from flowtoward the transducer array through one speaker and flow away from thetransducer array through a second speaker. The flow direction isconveniently given by the sign of the Doppler shift. In addition, theultrasound system of FIG. 1 enables the user to control the pitch of theaudio Doppler signal for better fidelity. For example, the loudspeaker44 may have poor reproduction fidelity of low frequency Doppler signalsfrom low velocity venous blood flow. In that case, the user wouldincrease the pitch of the reproduced Doppler sounds to a frequency rangewhich is more clearly reproduced by the loudspeaker.

In the example of FIG. 1 the ultrasound system has a control panel 20 bywhich the user can control a number of the Doppler features of theultrasound system. It will be appreciated that the control panel can beconstructed in hardware or as softkeys on a display screen or acombination of the two. A user control can be manipulated by the user tosteer a Doppler beam vector 22 over the image field. The vector 22 isgraphically produced as a line over the ultrasound image and is theindicator to the ultrasound system of the direction that the Dopplerbeam is to be transmitted for spectral Doppler interrogation. The usercan also manipulate a control of the control panel to position a samplevolume graphic 16 (see FIG. 2) at the depth along the Doppler beam wherespectral Doppler data is to be acquired. The Doppler beam vector and thesample volume location are coupled to the beamformer controller 60 toinform the controller of the direction in which the Doppler beam is tobe transmitted and the depth along the beam where spectral Dopplermeasurements are to be made. Gating of the Doppler demodulator 34 may beset by the same timing signals. In addition, the user can manipulate acontrol of the control panel 20 to indicate the direction of blood flowin a blood vessel with a flow cursor. This cursor setting is used by theultrasound system to correct the Doppler frequency estimate, as theDoppler equation used to estimate the Doppler frequency isangle-dependent upon the angle between the Doppler beam direction andthe direction of the blood flow. The beamformer controller also controlsDoppler transmission to be at a nominal Doppler transmit frequency ofthe transducer array, f₀. This information is typically supplied to theultrasound system by a memory device in the probe when the probe isconnected to the ultrasound system as described in U.S. Pat. No.4,868,476 (Respaut). In Doppler operation the probe 10 will transmit theDoppler beam at this nominal transmit frequency and the Doppler shift orfrequency will be offset from this nominal frequency.

In the example of FIG. 1, the nominal Doppler transmit frequency f₀, theDoppler beam vector setting, and the flow cursor setting are coupled toa pitch controller 52. In addition the pitch controller receives a pitchparameter m which is set by the user from the control panel. The pitchcontroller is then able to calculate the angle between the Doppler beamvector and the flow cursor and provide this angle in the form of cos Φto the spectral Doppler processor 36, which uses this term for anglecorrection of the Doppler estimates. The pitch controller 52 also usesthe data it receives to calculate a pitch scaling factor K for pitchcontrol. The factor K and the Doppler frequency f_(D) from the Dopplerfrequency estimator 34 are applied to a phase vocoder 50 for control ofthe pitch of the Doppler audio signal. Phase vocoders have been used inthe past for speech synthesis and music editing. However in theultrasound system of FIG. 1 the phase vocoder 50 is used for Doppleraudio by producing pitch-shifted signal components of the formI_(S),Q_(S), which are used by the forward/reverse separator 54 and theDAC 42 to drive the loudspeaker(s) 44 with a Doppler audio sounddifferent from that which is based on the transmitted Doppler frequencyf_(o).

A conventional Doppler ultrasonic imaging system produces a broadbandaudio signal whose frequency spectrum is related to the velocities ofmoving scatterers within a region of interest in the body through theDoppler equation. That is, the intensity of the audio signal at eachaudio frequency is proportional to the sum of the intensities of theacquired ultrasonic echoes from all scatterers moving at a velocity v,where f_(D) and v are related through the Doppler equation as:

$f_{D} = \frac{2v\; f_{0}\cos\;\phi}{c}$where f₀ is the ultrasonic demodulation center frequency (conventionallyequal to the transmit frequency) Φ is the Doppler angle, the anglebetween the Doppler beam direction and the direction of blood flow, andc is the speed of sound. Normally, once the Doppler frequency isestimated, the Doppler equation is used to calculate the blood flowvelocity. However, in an implementation of the present invention, thefrequencies of all components of the Doppler audio signal are scaledsuch that the intensity of the frequency-scaled audio signal at eachaudio frequency, f_(D′), is now proportional to the sum of theintensities of the acquired ultrasonic echoes from all scatterers movingat a velocity v, where f_(D′) and v are related through the followingequation:

$f_{D^{\prime}} = {K\frac{2v\; f_{0}\cos\;\phi}{c}}$where k typically is in the range of 1.0<=k<=4.

In the ultrasound system of FIG. 1, the phase vocoder 50 uses thequadrature Doppler components I₀′,Q₀′ used for spectral Doppler displayand the factor K supplied by the pitch controller 52 to produce a newsignal for Doppler audio reproduction in accordance with the equation:

$f_{D^{\prime}} = {K\frac{2v\; f_{0}\cos\;\phi}{c}}$The factor K in the equation will shift the pitch of the f_(D) Dopplerfrequency band to produce a Doppler audio signal f_(D′) with a differentpitch set by the pitch scaling factor K. The pitch scaling factor is setby the user's adjustment of the variable term m, where K=f(m). In aconstructed embodiment the term m can be sequenced over a range ofvalues with each discrete value producing a one-third octave shift ofthe Doppler audio sound. Six values are used so that the Doppler audiosound can be increased in pitch by up to two octaves. Adjustment of theuser control for Doppler audio pitch change will not affect the Dopplershift frequency values used for the visual spectral and colorflowDoppler displays, which produce their visual information using theunaltered f_(D) Doppler frequency.

The pitch controller 52 and the phase vocoder 50 can be used in otherimplementations to provide other benefits. For instance, as mentionedabove, since the Doppler equation contains the term f₀, the nominaltransmit Doppler frequency, the Doppler sound will depend upon thefrequency of the particular probe used for the exam. A 3 MHz probe willproduce a lower frequency sound than will a 5 MHz probe. The sonographermay have a discerning ear for 5 MHz Doppler audio, for example, and maywant the Doppler sound to be referenced to 5 MHz regardless of the probeor Doppler transmit frequency which is used for the exam. This can bedone by having the pitch controller 52 set K equal to:

$K = \frac{5\mspace{14mu}{MHz}}{f_{0}}$when the factor K is calculated in this manner, the Doppler equationwith the K factor becomes:

$f_{D^{\prime}} = \frac{2v\;\left( {5\mspace{14mu}{MHz}} \right)\cos\;\phi}{c}$The factor K thus causes probe-dependent frequency term f₀ to beeliminated and the frequency f_(D′) is always a function of a fixed 5MHz. Thus, the Doppler audio will always sound like that of a 5 MHzprobe. Doppler audio with a consistent pitch is produced for Dopplerprobes of different Doppler frequencies.

In another implementation the factor K may be calculated by the pitchcontroller 52 to produce Doppler audio sound which is invariant withchanges in the Doppler angle. This may be done by calculating K to be:

$K = \frac{1}{\cos\;\phi}$When this K factor is used in the Doppler equation the Doppler angle isremoved from the calculation of the Doppler audio frequency f_(D′).

It will also be appreciated that the two concepts above could be mergedto make the Doppler sound invariant to both changes in the Doppler angleand the frequency of the probe used. Embodiments of the presentinvention can shift the pitch of the audio Doppler signal, without anyalteration of the transmitted ultrasonic frequency or the Dopplerdemodulation frequency.

A typical Doppler display presented on the display 24 for simultaneouscolorflow and spectral Doppler interrogation is shown in FIG. 2. In thisdisplay the upper sector image 100 contains the B mode structuraldisplay overlaid with color information showing the blood flow. Thetransducer array is positioned at the apex 14 to acquire the image. Theblood flow through the vessel passing through the center of the sectorimage 100 may continually change from red to blue with the pulsatilechange in the blood flow velocity, for example. The user can position aDoppler beam direction line 22 over the image 100 with a user control tointersect the blood vessel at a point where a spectral Dopplermeasurement is to be made. The sample volume graphic 16 is then moved upand down the line 22 with a user control until it is positioned over theblood vessel where the spectral Doppler measurement is to be made. Thetilt of the flow cursor 18 is then adjusted by a user control toindicate the direction of blood flow, and the angle between the Dopplerbeam line 22 and the flow cursor 18 is the Doppler angle for anglecorrection. The resultant spectral Doppler display 120 of the blood flowat the sample volume 16 is shown at the bottom of the display screen.

The concept of shifting the pitch of the Doppler audio spectrum may beappreciated by referring to FIGS. 3a-3e which show a sequence ofspectral lines of a portion 70 of a spectral Doppler display. Eachspectral line of the display such as spectral line 70 is comprises of asequence of data points along the line. The position of a data point onthe line represents a velocity value as indicated by the ±V scale to theleft of the spectral lines, and each data point has a magnitude which isa function of the content of the frequency bin which provided thatvelocity. This spectral information may be represented in both frequencyand magnitude by the spectral band 80 drawn to the left of the spectraldisplay in FIG. 3b . As the curve 80 illustrating the envelope of thespectral band indicates, the spectrum has a particular shape determinedby the range of velocities of blood cells and by the presence ofpredominate velocities which are represented by increased amplitude ofthe curve (to the left in the drawings).

A simple way to change the Doppler sound for a higher frequency responseloudspeaker is simply to shift the frequency of the spectrum 80 to ahigher frequency band 80′ as shown in FIG. 3c . The sound will now bereproduced at the higher frequencies of the shifted band 80′. But thissimple frequency shift will not reproduce the timbre of the sound. Thehigher frequency sound will be metallic and inharmonic and will sounddissonant to the ear of the trained sonographer. To prevent this animplementation of the present invention will shift the pitch of thesound, changing the spectrum 80 of FIG. 3b to the pitch-shifted spectrum82 of FIG. 3d . In this new spectrum the frequency components have beenstretched or dilated, and harmonic relationships retained. It is seenthat the shape of the spectral envelope is preserved but stretched.Thus, a 100 Hz component is scaled to 200 Hz, a 200 Hz component isscaled to 400 Hz, a 300 Hz component is scaled to 600 Hz, and so on,which preserves the harmonic content and the timbre of the sound. Thiseffect is shown in a normalized logarithmic frequency scale in FIG. 3e ,where the conventional audio spectrum Sa is shifted by one octave to apitch-shifted spectrum Sa′. It is seen that the spectrum appearsunaltered in shape, but is shifted up in pitch by one octave. Thepitch-shifted Doppler audio will appear to the sonographer to be thesame Doppler sound as before, but at a higher pitch. And when the higherpitch is better aligned with the passband of the loudspeaker, a clearerand more distinct Doppler audio sound is produced by the ultrasoundsystem.

There are a number of techniques for shifting the pitch of the Doppleraudio signal such as time domain harmonic scaling, wavelet processing,and use of a phase vocoder. The phase vocoder approach is preferred asphase vocoders are well understood in other applications and a phasevocoder can be implemented with overlapping short-time FFT processing,which is a common approach for Doppler spectral signal processing. Thephase vocoder, which can be implemented in hardware or algorithmicallyby software processing, models the audio signal as a set of narrowbandtones, one for each FFT frequency bin. The change of magnitude of an FFTbin between successive overlapping FFTs is interpreted as a gradualchange of amplitude of the narrowband tone over that short span of time.The change of phase of an FFT bin between successive overlapping FFTs isinterpreted as a precise frequency adjustment of the narrowband toneover that short span of time, a perturbation from the center frequencyof the FFT bin. Each frequency-adjusted tone is then used to interpolatethe corresponding FFT bin in time with intermediate magnitude and phasevalues, producing more overlapping FFTs for the same original time span.These overlapping values are processed by inverse-FFT processing andoverlapped-added (with the original overlap factor) in the usual way ofshort-time FFT reconstruction, producing more samples than in theoriginal sampled signal. If replayed at the original sample rate, thiswould produce time-stretched audio at the original pitch. But ifreplayed at a faster sample rate to match the original time span, thepitch is scaled up by the FFT interpolation factor. The pitch or timecan be scaled down by decimating instead of interpolating the FFTs.

Phase vocoder processing is illustrated by FIG. 4 as one possibleimplementation of pitch shifting. In this example the frequencies willbe doubled, shifting the pitch up one octave. The small vertical lines90 represent a sequence of Doppler audio samples before pitch shifting,with time progressing in the horizontal direction and the spacingbetween the samples representing the sample rate. The sequence ofsamples can be segmented in overlapping windows W as shown by thebrackets above and below the sequence of samples 90. In thisillustration each window contains eight samples and is 50% overlappedwith adjacent windows, although windows with greater or fewer samplesand more or less overlap can be used.

Each segmented window of time samples is multiplied by a smoothlytapered window function, then converted to frequency domain with an FFT(Fast Fourier Transform) in step 902. The smoothly tapered windowfunction is not shown in the figure, but is a standard step in FFTprocessing to reduce time discontinuity and frequency smearing caused bythe FFT treating the time segment as a periodic function. The result ofthe FFT of successive windows is a time sequence of frequency domaincomplex spectra, represented by the vertical sets of small horizontallines 96 in the figure. Each sample (called a “bin”) of each spectrum isa complex number, whose magnitude and phase correspond to a windowedsinusoid at the center frequency of the bin. Considering the samefrequency bin in two successive complex spectra, the difference in phaseover that increment of time can be interpreted as a slight frequencyoffset from the center frequency of a bin, since frequency is the timederivative of phase. In other words, the center frequency (phase rate)of the bin produces a deterministic large change of phase over the timeinterval between spectra, and the actual phase difference between thespectra is used as a slight adjustment to this phase rate. Phasecalculations are modulo 2π radians (360 degrees).

Using the slightly adjusted frequencies and the magnitudes for each binin successive spectra, additional complex spectra 98 are interpolated asshown in the figure and indicated by step 904. In this example, thenumber of spectra over a span of time is doubled, corresponding to anaudio frequency scaling of two, which is a one octave increase in pitch.The complex spectra are then converted back to the time domain with aninverse FFT, producing segmented windows W′ of time samples 92, whichare added together with the same overlap fraction (50% in this example)as in the earlier window and FFT processing step 902. This isillustrated in the figure with the small vertical lines 92 and bracketsW′. Since the interpolation between spectra produced more spectra perunit of time, after the inverse FFT and overlap add there are moresamples per unit of time. The frequency content of the data isequivalent to the original audio signal in terms of samples, but thesample rate now higher (doubled in this example). So when the data areconverted to a continuous analog signal with the new sample rate andapplied to a loudspeaker, all of the frequencies are scaled by the ratioof new to original sample rates.

The frequency scale factor is not constrained to an integer, because thecomplex spectra can be resampled to any rate. For the time samples toalign in the overlap-add step 906 following the inverse FFT, the scalingfactor should be a ratio of integers, where the denominator is thenumber of samples in the window. However, even this constraint can beeasily overcome by multiplying each interpolated frequency spectrum witha linear phase ramp corresponding to its fractional sample time shift.So there is essentially no constraint on the quantization of frequencyscale factor.

The primary trade-off in the phase vocoder processing is the FFT size.Many small FFTs can better follow temporal transients, but only have afew frequency bins, which can be audible as distinct tones. Few largeFFTs can better estimate a continuous frequency spectrum, but have aslow temporal evolution that can be audible as phase smearing. Theartifacts become more noticeable or objectionable as the pitch scalefactor increases. Thus, the preferred embodiment uses medium FFT sizescorresponding to about 20-30 msec.

If the sample sequence 94 of FIG. 4 were played at the original samplerate of FIG. 4 through the DAC 42 and loudspeaker 44, the audio signalwould be a time-stretched version of the audio signal of the sequence 90of FIG. 4 at the same pitch as the FIG. 4 sequence. But when thesequence 94 is played at a higher sample rate so that each window at thebottom of FIG. 4 is played in the same time interval as a window at thetop of FIG. 4, the pitch is scaled up. In the illustrated example wherethe number of samples in each window is doubled, each window of samplesin FIG. 4 is then played at twice the sample rate of FIG. 4 and thepitch is scaled up by a full octave. If the FFT processing is used todecimate the sample sequence instead of upsampling it, the pitch wouldbe scaled down instead of up.

It is seen in FIG. 4 that there is a ratio of the number of samples 90in the windows W of the original sequence to the number of samples 94 inthe windows of the final sequence at the bottom of FIG. 4. This ratiocan be expressed as a ratio of integers. In this example the ratio is8:16 or 1:2. The user control of the control panel which adjusts thepitch of the Doppler audio can provide one of these integers, m.

What is claimed is:
 1. An ultrasonic diagnostic imaging system whichproduces a Doppler audio signal of measured blood flow, the systemcomprising: an ultrasound probe, operating at an ultrasonic Dopplertransmit frequency f₀, configured to acquire Doppler ultrasound signalsin a range of frequencies in a Doppler audio band, said ultrasoundsignals being referenced to the Doppler transmit frequency from alocation of blood flow; a Doppler demodulator, responsive to the Dopplerultrasound signals, configured to produce Doppler shift signals of thevelocity of blood flow in an audio frequency band; a Doppler informationdisplay, responsive to the Doppler shift signals, configured to displaymeasured blood flow velocity based upon the Doppler shift signals; andan audio Doppler system, responsive to the Doppler shift signals,configured to produce pitch-shifted audio Doppler without changing thedisplayed blood flow velocity, wherein the audio Doppler system isresponsive to a user control to shift the pitch of the Doppler-shiftedsignals by a fractional or integer number of octaves by stretching ordilating the entire range of frequencies within the Doppler audio bandand retaining harmonic relationships in the Doppler audio band so as topreserve the timbre of audio Doppler sounds, wherein the audio Dopplersystem is responsive to the user control to produce audio Doppler soundsinvariant to changes of a Doppler angle or based on an indication of adirection of blood flow adjusted by a user.
 2. The ultrasonic diagnosticimaging system of claim 1, wherein the audio Doppler system is operativeto produce pitch-shifted audio Doppler without change in the ultrasonictransmit frequency f₀ or the displayed blood flow velocity.
 3. Theultrasonic diagnostic imaging system of claim 2, wherein the usercontrol is operable to control a scaling of the frequency band for audioDoppler.
 4. The ultrasonic diagnostic imaging system of claim 2, whereinthe audio Doppler system further comprises a phase vocoder which shiftsthe pitch of the Doppler-shifted signals.
 5. The ultrasonic diagnosticimaging system of claim 2, wherein the audio Doppler system furthercomprises a pitch controller, responsive to the user control, whichproduces a pitch shift factor K for control of the audio pitch shift. 6.The ultrasonic diagnostic imaging system of claim 1, wherein the audioDoppler system further comprises a loudspeaker exhibiting a givenfrequency response; and wherein the shift in pitch is determined by thefrequency response of the loudspeaker.
 7. The ultrasonic diagnosticimaging system of claim 1, wherein the Doppler demodulator is operativeto produce demodulated Doppler signals in a band exhibiting ademodulation reference frequency, which band is determined by a Dopplerequation, wherein the pitch-shifted audio Doppler band is different fromthe band determined by the Doppler equation and the demodulationreference frequency.
 8. The ultrasonic diagnostic imaging system ofclaim 7, wherein the Doppler equation is of the form$f_{D} = {\frac{2{vf}_{0}\cos\;\varnothing}{c}.}$
 9. The ultrasonicdiagnostic imaging system of claim 1, wherein the imaging system isoperable with a selected one of a plurality of ultrasound probes, eachexhibiting a different nominal Doppler frequency f₀.
 10. The ultrasonicdiagnostic imaging system of claim 9, wherein the Doppler demodulator isoperable to detect Doppler signals by a Doppler equation which is afunction of f₀ of a selected probe; and wherein the audio Doppler systemproduces audio Doppler related to a selected f₀ frequency for probes ofdifferent nominal Doppler frequencies.
 11. A method of producing aDoppler audio signal of measured blood flow, the method comprising:using an ultrasound probe, operating at an ultrasonic Doppler transmitfrequency f₀, acquiring Doppler ultrasound signals in a range offrequencies in a Doppler audio band, said ultrasound signals beingreferenced to the Doppler transmit frequency from a location of bloodflow; based at least partially on the Doppler ultrasound signals,producing Doppler shift signals of the velocity of blood flow in anaudio frequency band; displaying measured blood flow velocity based atleast partially upon the Doppler shift signals; producing pitch-shiftedaudio Doppler without changing the displayed blood flow velocity; and inresponse to a user control, shifting the pitch of the Doppler-shiftedsignals by a fractional or integer number of octaves by stretching ordilating the entire range of frequencies within the Doppler audio bandand retaining harmonic relationships in the Doppler audio band so as topreserve the timbre of audio Doppler sounds, wherein in response to theuser control, the audio Doppler sounds are produced invariant to changesof a Doppler angle or based on an indication of a direction of bloodflow adjusted by a user.
 12. The method of claim 11, wherein theproducing further comprising producing pitch-shifted audio Dopplerwithout change in the ultrasonic transmit frequency of the ultrasoundprobe.
 13. The method of claim 12, further comprising scaling of thefrequency band for audio Doppler in response to the user control. 14.The method of claim 12, wherein shifting the pitch of theDoppler-shifted signals is performed with a phase vocoder.
 15. Themethod of claim 12, further comprising producing a pitch shift factor Kfor control of the audio pitch shift.
 16. The method of claim 11,further comprising determining the shift in pitch based at leastpartially on a frequency response of a loudspeaker on an ultrasoundsystem.
 17. The method of claim 11, further comprising producingdemodulated Doppler signals in a band exhibiting a demodulationreference frequency, wherein the band is determined by a Dopplerequation, and wherein the pitch-shifted audio Doppler band is differentfrom the band determined by the Doppler equation and the demodulationreference frequency.
 18. The method of claim 17, wherein the Dopplerequation is of the form$f_{D} = {\frac{2{vf}_{0}\cos\;\varnothing}{c}.}$